Addition of rubbery impact modifiers is commonly used to improve the toughness of aromatic carbonate polymer compositions. In comparing the effectiveness of various rubbery modifiers large differences in melt stability are often observed. In aromatic carbonate polymer compositions, the polycarbonate portion of the composition is susceptible to degradation by acids and bases. Sufficient degradation will cause color formation, loss of molecular weight, reduced chemical and mechanical properties and generation of carbon dioxide which causes surface appearance defects, such as splay in molded parts.
The problem of the melt stability of polycarbonates and blends thereof with other polymers are often addressed by the addition of various stabilizers as disclosed in copending application Ser. No. 08/79112 filed Jan. 30, 1997 and assigned to the same assignee and in U.S. Pat. Nos. 4,532,290, 5,252,536, 5,441,997, 5,502,119 and 5,608,027 all of which are incorporated herein by reference. Rubbery modifiers prepared by emulsion polymerization are useful in polymer blends as impact modifiers due to their fixed particle size. However, they may cause particular problems in the stability of polycarbonates due to the presence of significant amounts of additives employed in preparing the rubbery modifiers such as flow aids, polymerization catalysts, antioxidants and especially emulsifiers. Some of the most common impact modifiers employed in the preparation of a carbonate polymer compositions are polymers themselves prepared by: polymerization techniques employing particular emulsifiers that are utilized in emulsion polymerization reactions; suspension agents utilized in suspension polymerization reactions; surfactants utilized in phase transfer catalysis systems; soluble catalysts utilized in polymerization reactions; and for certain process stabilizers employed during the processing of the polymers either in the polymerization or in the molding thereof. For example, it is now known that residues in an impact modifier from these particular polymerization aids, particularly emulsion polymerization aids employed in preparing certain impact modifiers, quite often catalyze transesterification or degradation of polycarbonates. It is also known that impact modifier additives to carbonate polymer compositions are commonly made by emulsion polymerization processes employing alkali metal salts of fatty acid emulsifiers to stabilize the emulsion during polymerization of the impact modifier. It is also known that the alkali metal salts of fatty acids catalyze transesterification or degradation of polycarbonates which in turn causes inconsistent thermal stability in carbonate polymer composition because of residual amounts of the emulsifier employed in preparing the modifier remaining in the impact modifier. This leads to problems in molding shops because of the variability in viscosity due to the catalytic transesterification or degradation of polycarbonates with such impact modifiers.
U.S. Pat. No. 5,608,027 Table 11 describes the superior performance of complex stabilization systems when impact modifiers are used in polycarbonate compositions and demonstrates problems that can occur with reaction between the polycarbonate and the impact modifier, if complex stabilizers are not added to prevent the reaction.
A large number of patents exists describing the use of additives in polycarbonate compositions. A significant fraction of these are directed to the use of impact modifiers. The instant invention discloses the utilization of certain impact modifiers prepared by emulsion polymerization in carbonate polymer compositions, which impact modifiers are essentially free of particular residual emulsifiers that cause degradation of polycarbonates. The instant invention is directed to an impact modified carbonate polymer composition with better thermal stability than compositions with conventional impact modifiers which employ, for example, alkali metal salts of fatty acid as emulsifiers in the preparation thereof.